A converter connected to an a.c. network, for example a converter included in a converter plant for high-voltage direct current, generates, by its principle of operation, harmonic currents on its a.c. side and harmonic voltages on its d.c. voltage side. In this context, in principle, only harmonics to the fundamental frequency of the a.c. network of the orders n=kp.+-.1 occur on the a.c. side and of the orders n=kp on the d.c. voltage side, p being the pulse number of the converter and k being a positive integer. Harmonics of other orders may also occur in power networks of this kind, caused by, for example, unsymmetries between the phases of the a.c. network.
To reduce the stresses on components included in the power network, and originating from the harmonics, and to fulfil the requirements made on the effect on the network and telecommunication disturbances, shunt-connected filters are therefore generally installed to limit the propagation of the disturbances in the power network. Harmonics of a lower order, for example those which correspond to k=1 and for 6-pulse converters also k=2, are generally filtered through filters tuned to these harmonics whereas harmonics of a higher order may be filtered through a high-pass filter. The filters are composed of passive components, and during the dimensioning it is also taken into consideration that the filters on the a.c. side are to serve as members for generating reactive power. In certain cases, it may be necessary to install tuned filters and high-pass filters also on the d.c. voltage side of the converter. In a converter plant for high-voltage direct current, these filters and the capacitor banks constitute plant components which considerably influence the function, volume and cost of the plant.
The tuned filters are generally designed as series-resonance circuits, comprising capacitive, inductive and sometimes also resistive impedance elements, so chosen that, at one or more of the harmonic frequencies expected in the power network, they are to exhibit a purely resistive impedance. These filters are connected between two conductors in the power network, of which one conductor may be galvanically connected to ground or consist of ground.
In, for example, 12-pulse converters, filters are usually installed on the a.c. side for filtering of at least the 11th and 13th tones. These filters may then be formed as two separate parallel-connected filters, each one essentially consisting of a series-connection of a capacitive, an inductive and a resistive impedance element. The filters are each tuned to one of the two above-mentioned tones, so-called single-tuned filters. Alternatively, the desired filtering may be achieved by means of a double-tuned filter tuned to both the 11th and 13th tones. In this way, the advantage is achieved, among other things, that only one capacitive impedance element need be dimensioned for full voltage stress. For a general discussion of this technical field, reference is made to E. W. Kimbark: Direct Current Transmission, John Wiley & Sons, Inc., 1971, in particular pages 363-367. For frequencies near the resonance frequencies, a double-tuned filter is substantially equivalent to two parallel-connected single-tuned filters. Relations for transformation of such a parallel connection of two single-tuned filters into a double-tuned filter are given, for example, in B. J. Cory (editor): High Voltage Direct Current Convertors and Systems, MacDonald & Co. Ltd., 1965, pages 154 and 174.
The above known configurations of double-tuned filters, however, exhibit certain drawbacks. The number of impedance elements remains large and comprises, inter alia, two inductive elements, between which magnetic coupling must be avoided. This means that the filter, at the site of the plant, requires relatively large ground space. The inductive impedance elements are not both located at any of the tapping points of the double-tuned filter. Further, the above-mentioned transformation method predetermines the impedance levels in the double-tuned filter, and therefore current and voltage utilization of the impedance elements therein cannot be freely optimized.